This invention relates to the froth flotation of coal-containing ashes, coal sludge or coal-containing residues to recover coal containing a lower percentage of impurities. In particular, this invention relates to the use of a condensation product of a fatty acid or fatty acid ester and a N-hydroxyalkylated, nitrogen-containing compound as a conditioner for the flotation of finely divided coal.
The natural process of "coalification" inherently deposits some non-combustible mineral matter in association with the combustible carbonaceous solids. Large fragments of non-combustible material can be removed by screening or other gravity concentration techniques, but other cleaning methods more efficiently remove fine material intimately associated with the carbonaceous solids. Froth flotation of coal is used in the art to beneficiate finely-divided raw coal. Bituminous coals generally possess a natural hydrophobicity, which results in the coal being floatable in the presence of a frother, such as methyl isobutyl carbinol, desirably with a relatively mild collector, such as kerosene. However, anthracite coals, as well as coals of all ranks in which the surface has been at least partially oxidized, float poorly in such a medium, resulting in the loss of significant amounts of combustible material with the tail fraction from the flotation.
The loading of the oil-type collector is generally 0.1 to 2 pounds per ton of coal feed for bituminous coals of intermediate or low rank, with the loading being relatively greater for the flotation of lignite and anthracite coals. However, good recovery of oxidized coals or lignite coals can only be effected at such high loadings of the oil-type collector that significant amounts of inert material are floated along with the combustible materials. Sun suggests in Trans. AIME, 199:396-401 (1954), that fatty amines can be utilized as co-collectors in the flotation of oxidized coals to effect enhanced recovery. However, even these amine collectors float substantial amounts of ash along with the coal and effect only partial recovery of combustible material.